Treadwell said:
From what I've read, that was not the case in those days--that existing mono projectors could not read the stereo prints' dual optical track. That was the reason they did a mono mix in the first place.
Yeah, I'm not entirely sure but here's how in70mm describes the issue,
Although the 35mm Dolby Stereo process was designed to be (and is) mono-compatible, at the time, those involved with the new technology were concerned about the effectiveness of mono playback from a stereo-encoded print for both technical and aesthetic reasons. For similar reasons, a decision was made not to create the mono master by means of dubbing the stereo master and "folding" the multiple tracks into one. Instead, a new dedicated mono mix was created from scratch.
Treadwell said:
Speaking of opening day, I thought that only 75mm houses were running the film on May 25.
Both 35mm Dolby and 70mm Dolby presentations was available opening day and apparently monaural and perhaps regular stereo presentations where Dolby was not yet available, here's a list on the first-week engagements: http://www.in70mm.com/news/2003/star_wars/chapters/theatres.htm